In general, in a fuel cell generating system formed by combining a fuel cell and a fuel reformer, it is difficult to have the output of the fuel cell respond quickly with the variation in load when the load is varied suddenly under operating conditions of traveling or cargo loading since the response speed of the reformer is slow. Such being the case, a fuel cell generating hybrid system, in which a storage battery is connected to the output side of the fuel cell as a backup for the fuel cell, has been used, in which needed electric power not capable of being supplied by the fuel cell is discharged from the storage battery under heavy load operating conditions, and in which the storage battery is charged for recovery with surplus electric power of the fuel cell under light load or no-load operation, as proposed by the present applicant under Japanese Patent Application No. 121147/87.
On the other hand, when the storage battery continues to be charged from the fuel cell under light load or no-load operation for a long time, the storage battery is over-charged. Conversely, when the shortage in electric power of the fuel cell is discharged from the storage battery for a long time under heavy load operation, the storage battery is over-discharged. If such overcharged and overdischarged states are repeated, the life of the storage battery is extremely shortened. A control unit of a fuel cell generating system, in which the charged quantity of the storage battery is monitored, the storage battery is held under a highly charged state so as to cope with a heavy load by controlling the output of the fuel cell in correspondence with the residual capacity of the storage battery, and the stabilization of electric power fed to the load is devised at the same time, has been proposed as a countermeasure by the present applicant under Japanese Patent Application No. 110634/87.
Thereupon, it is required to always maintain the storage battery of the generating system always under a state of high residual capacity so as to cope with a heavy load in case that the load is fluctuated suddenly because of cargo handling or travel motion such as in the above-mentioned case of a forklift truck. To this end, it is required to finish charging the storage battery in shortest possible time within the window of an allowable charging condition so as not to shorten the life of the storage battery.
The above-mentioned proposed control systems utilize a basic control system that controls and varies only the output of the fuel cell in correspondence with the charged quantity of the storage battery. They are insufficient to finely control the output of the fuel cell based on the charging characteristics of the storage battery to finish charging for recovery of the storage battery in the shortest possible time under allowable charging conditions, in such a manner that does not cause deterioration of performance of the storage battery as described above.
With reference to FIG. 3, there is shown charging and discharging characteristics at a regular time factor of the storage battery with a lead storage battery as an object. In regions A and B of said characteristics, the battery reactions of charging and discharging are performed reversibly, whereas the stable reversible reaction is collapsed in the overcharged or over-discharged state shown in region C and an irreversible state is produced. It has been known that, if the storage battery is used repeatedly in this region, the battery characteristics are not only deteriorated, but also the life is shortened sharply. Also, FIG. 4 shows current to voltage characteristics in the charging of a lead storage battery in terms of respective discharged quantities (%), and a characteristic line D in the Figure represents a boundary line of the allowable charging region. That is, safe charging operation is performed with a small charging current below the intersection of characteristic lines of respective charging quantities (%) and the boundary line D of the charging allowable regions, but deterioration of performance of the storage battery progresses rapidly if rapid, which applies a large charging current, is repeated in the broken line region which exceeds the intersection of said characteristic line. In other words, when a storage battery is charged for recovery, the allowable charging current, thus the allowable charging voltage, is different in accordance with the state of residual capacity from the characteristic aspect of the storage battery. In particular, when the light load operating time in the intervals of heavy load operation is utilized, as mentioned above and it is required to charge the storage battery for recovery in a short time without causing deterioration of battery characteristics, fine control of performing charging for recovery under charging conditions closest to the very limit of the allowable region shown in FIG. 4 is required.
It is an object of the present invention to provide a control unit of a fuel cell generating system which makes it possible to perform fine control so as to charge the storage battery for recovery within the shortest possible time to reach the target charged quantity under such charging conditions that do not cause deterioration of performance of the storage battery, and which exhibits excellent effects in a fuel cell generating hybrid system used as a power supply in applications subject to sudden load fluctuations such as are encountered in a fork lift truck, for example.